Determination of a corneal thickness is important for various diagnostic applications as well as an important part of various surgical interventions on the eye. A method and apparatus for obtaining such a thickness is e.g. known from U.S. Pat. No. 6,692,126. The apparatus disclosed applies a Placido ring illuminator to illuminate the cornea and derives a model of the anterior surface of the cornea from the image obtained. By projecting thin slits of light on the cornea, a second image can be taken and used to determine the corneal thickness from the model of the anterior surface and the second image.
The apparatus and method as known in the art suffers from a number of drawbacks. First of all, this system needs to focus on two different planes: firstly the iris plane for imaging the reflections of the Placido illuminator, and secondly the cornea plane for imaging the cornea, which is partially illuminated by the slits of light. This system therefore requires the use of two camera's or one camera with a rapidly moving lens. This will drive up complexity and costs of the camera.
Second, it is known that the use of a Placido ring illuminator requires some assumptions with respect to the corneal anterior surface. Due to the use of ring shaped light sources, a one-to-one correspondence between a point on the stimulator source (i.e. the Placido ring illuminator) and a point on the captured image cannot be determined unless certain symmetries in the corneal surface are assumed. As in reality, these assumed symmetries may not be present; inaccuracies may be introduced in the model. As the model is further on applied to determine the corneal thickness, inaccuracies in this thickness due to the assumed symmetries of the anterior surface may occur as well. It can further be noted that the use of slit-shaped illumination sources results in a similar problem in that a one to one correspondence between a point on the stimulator source (i.e. the slit-shaped illumination source) and a point on the captured image may be difficult to establish. A further drawback of the apparatus as known in the art is the requirement of sequentially capturing two images of the cornea to determine the corneal thickness. In case of a displacement of the eye between the capturing of the first and the second image, some uncertainty with respect to the position of the anterior surface of the cornea may exist when the second image is taken. This uncertainty may further introduce inaccuracies in the determination of the corneal thickness. As an alternative, it is proposed in U.S. Pat. No. 6,692,126 to use a camera system with multiple camera's each camera being arranged to record an image of one of the illumination sources. This may result in a more complex and therefore more expensive apparatus.
It is further acknowledged that other apparatuses exist for approximating a corneal thickness. One of such apparatuses is described in Optometry and Vision science, Vol. 67, No. 10, pp. 757-763 and uses a plurality of stimulator points for illuminating the cornea. The image obtained is used to estimate the corneal thickness by assuming that both the anterior surface and the posterior surface are spherical surfaces.
It is an object of the present invention to provide an apparatus for corneal diagnosis and a method for determining a corneal thickness that alleviates, at least partly, one or more of the drawbacks mentioned above.